1. Field of the Invention
This invention relates to a lithography technique for use in the production of solid state devices.
2. Description of the Prior Art
The production of solid state devices, including semiconductor integrated circuits and discrete devices, lightwave devices, Josephson junction devices, magnetic bubble devices, etc., typically requires the generation and replication of fine line patterns during at least some stages of the production process. Higher density circuits and higher performance devices are achieved in part by reducing the size of the geometry of the patterns. Current research is typically concerned with generating patterns having minimum line widths of 2 microns or less and especially with dimensions of 1 micron or less.
Lithographic materials utilized for recording pattern information include organic polymers sensitive to visible light, ultraviolet light, X-ray radiation, or electron beam radiation. Ion beams have also been considered for use in pattern generation. Most present day resists known to be suitable for irradiation by electron beams are also suitable for irradiation by ion beams. For example, poly(methylmethacrylate) (PMMA) is a standard resist utilized in research on submicron geometry devices. When exposed to an electron beam, PMMA and certain other electron beam resists undergo band scission in the irradiated region, rendering the irradiated region more soluble in a developer, producing a positive tone resist pattern. The pattern is then typically transferred to an underlying layer, such as an oxide layer, by etching techniques known in the art. Ion beams can offer even higher resolution than electron beams, due to the higher mass of the accelerated particles and the shorter range of the secondary electrons produced upon impact. Ion beams are generally thought to expose PMMA and other electron beam resists mainly by these secondary electrons. Thus, the chemistry of the resist irradiation process is similar for both electron beam irradiation and ion beam irradiation for most known resists.
One problem of irradiating known electron beam resists with heavy ion beams (i.e., ions of elements having an atomic weight greater than about 20) is that the ion beams typically penetrate only a short distance into the resist material, leaving a large part of the resist substantially unexposed. For example, in a 1 micron layer of PMMA, gallium or indium ion beams accelerated through a potential of 20 kilovolts penetrate only about 250 Angstroms into the resist. Upon wet development, the regions underlying the exposed layer are not fully developed, leaving only a shallow relief image in the exposed regions. While higher energy ion beams can be used to overcome this problem, such beams are currently difficult to implement in scanning ion beam systems.
Therefore, it is desirable to obtain resist materials that can be exposed with relatively low energy ion beams while achieving fine line pattern generation. In other areas of concern, it is desirable in some cases to eliminate polymeric resist materials entirely and to form a pattern directly on a device layer; for example, a silicon dioxide layer or a metal layer.